Update README.md

Author: jurgenvinju

README.md

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 # rascal-core
 
+> *warning*
+> this project's repository has been merged into the rascal project.
+> http://github.com/usethesource/rascal
+>
+
 Core components of the Rascal meta-programming language (in particular run-time, type-checker and compiler).
 
 * this is a temporary project to keep the bootstrapping process clear and separate from the current implementation in the https://github.com/usethesource/rascal project